We present CosmoGuesser, an inquiry-based, game-based learning puzzle board game for secondary astronomy education, developed within the ASTRO-EDU Erasmus+ project. Its core mechanic links gameplay directly to astrophysical reasoning: players use real observational signatures from six Caldwell catalog sky zones to deduce object identity through tiered information cards and a "Hypothesis Marker" mechanic rewarding reasoning under uncertainty. The puzzle itself enacts inquiry: players form hypotheses, test them against evidence, and revise as new clues appear. We discuss this design framework and welcome feedback before prototyping begins.